The present invention relates to a method for producing a honeycomb structure and a honeycomb structure.
Porous honeycomb structures comprising cell partition walls (ribs) forming a composite of a plurality of cells adjacent to each other and a honeycomb outer wall enclosing and holding outermost cells positioned at outermost periphery of the cell composite are widely used as a filter for catching and removing particulate materials contained in dust-containing fluid such as diesel engine exhaust gas (diesel particulate filter (DPF)) or a catalyst carrier (for purification of exhaust gas) which supports a catalyst component for purification of exhaust gas to remove harmful materials contained therein, and as the materials constituting the honeycomb structures, there are used refractory silicon carbide (SiC), cordierite and composites thereof, etc.
Furthermore, development of DPF is advanced in which a regeneration system is employed which carries out continuous regeneration by supporting an oxidation catalyst on a conventional DPF and oxidizing and burning the deposited particulates (DPF for regeneration of catalyst).
For economical production of the above honeycomb structures, it is preferred from the points of improvement of yield and reduction of cost to prepare (recycle) starting raw materials from recovered materials which originate from starting raw materials for honeycomb structures and which are produced during production of honeycomb structures (for example, unfired dried products such as unfired molded products removed between drying step and firing step for some reasons or fragments thereof (including fired products in the case of SiC)) and reuse them for molding as recycled raw materials.
In the case of cordierite ceramic materials, when unfired dried products which are recovered are ground for using as recycled raw materials, the cordierite ceramic materials produced using the ground powders are, in many cases, greater in coefficient of thermal expansion than cordierite ceramic materials produced using inherent starting raw materials and are deteriorated in thermal shock resistance, and hence they cannot be used as honeycomb catalyst carriers for purification of exhaust gas.
In order to solve the above problems, there are proposed cordierite ceramic materials and methods for producing the same, for example, a method of crushing, grinding and classifying unfired dried products which are recovered materials at light grinding step to prepare (recycle) raw materials and using the recycled raw materials (Japanese Patent No.1703709), a method of removing at least a part of binder contained in unfired recycled raw materials, then grinding the recycled raw material to prepare a ground powder, adding water, binder, etc. to the ground powder, followed by kneading to prepare a recycled clay, and molding and firing the recycled clay (JP-A-2000-302533), and a method of grinding unfired recycled raw material to prepare a ground product, removing powders of below 1 mm in diameter from the ground product, and adding water to the remaining ground product, followed by kneading to prepare recycled clay, molding the recycled clay and firing the molded product (JP-A-8-119726).
However, according to the method of Japanese Patent No.1703709, when the recycled raw materials are ground to a particle size of below 10 μm, not only the grinding efficiency is low to require much cost, but also abrasion of the grinding equipment is great and a large amount of impurities are incorporated.
According to the method of JP-A-2000-302533, since the recycled raw materials are heat treated to remove binder, the production cost is high, and according to the method of JP-A-8-119726, since recycled materials of 1 mm or less are removed, yield of the recycled raw materials is low and the recycled materials are coarse, and hence the dissolution of raw materials at kneading becomes insufficient, resulting in clogging at the time of molding of honeycomb.
Furthermore, in the case of producing a DPF, a coarse raw material must be used for increasing the porosity, but if a ground recycled material containing beyond 20% by weight of particles having an average particle diameter of than 45 μm or less is used, the average pore diameter of the resulting DPF becomes small.
Moreover, in the case of producing a DPF, if a ground recycled material containing beyond 10% by weight of particles having an average particle diameter of 2800 μm or more is used, the recycled raw material is not sufficiently dissolved at the steps of mixing and kneading, and there are possibilities of causing clogging of die during molding to result in defect of cells in the resulting honeycomb structure.
The present invention has been made in view of the above problems in the conventional technologies, and the object of the present invention is to provide a method for producing a honeycomb structure using an unfired recycled raw material (which may be a fired material in the case of SiC) recovered in the course of production of honeycomb structure, wherein the resulting honeycomb structure has low thermal expansion and porosity similar to those of a honeycomb structure obtained using inherent starting raw materials and yield of the honeycomb structure can be improved and production cost can be considerably reduced, and another object is to provide the honeycomb structure.